There are two principal components of sleepiness or fatigue:
• Physiological sleepiness or fatigue—this is a requirement like hunger or thirst and can only be reversed by sleep.
• Subjective sleepiness or fatigue—this is an individual's perception of his or her sleepiness but it may be affected by other factors. It may be difficult for an individual to subjectively assess his or her own alertness, with a tendency to report a greater level of alertness than is actually the case.
Factors affecting sleepiness include:
• Prior sleep and wakefulness
• Circadian phase leading to:
increased sleepiness in the early hours of the morning and during the afternoon decreased performance in the early hours of the morning
• Age (the amount of nocturnal sleep required reduces after the age of 50)
• Alcohol (reduces the quality of sleep)
• Work and environmental conditions.
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